The present invention relates to combustors for gas turbine engines in general, and to heat shield panels for use in double wall gas turbine combustors in particular.
Gas turbine engine combustors are generally subject to high thermal loads for prolonged periods of time. To alleviate the accompanying thermal stresses, it is known to cool the walls of the combustor. Cooling helps to increase the usable life of the combustor components and therefore increase the reliability of the overall engine.
In one cooling embodiment, a combustor may include a plurality of overlapping wall segments successively arranged where the forward edge of each wall segment is positioned to catch cooling air passing by the outside of the combustor. The forward edge diverts cooling air over the internal side, or hot side, of the wall segment and thereby provides film cooling for the internal side of the segment. A disadvantage of this cooling arrangement is that the necessary hardware includes a multiplicity of parts. There is considerable value in minimizing the number of parts within a gas turbine engine, not only from a cost perspective, but also for safety and reliability reasons. Specifically, internal components such as turbines and compressors can be susceptible to damage from foreign objects carried within the air flow through the engine.
A further disadvantage of the above described cooling arrangement is the overall weight which accompanies the multiplicity of parts. Weight is a critical design parameter of every component in a gas turbine engine, and that there is considerable advantage to minimizing weight wherever possible.
In other cooling arrangements, a twin wall configuration has been adopted where an inner wall and an outer wall are separated by a specific distance. Cooling air passes through holes in the outer wall and then again through holes in the inner wall, and finally into the combustion chamber. An advantage of a twin wall arrangement compared to an overlapping wall segment arrangement is that an assembled twin wall arrangement is structurally stronger. A disadvantage to the twin wall arrangement, however, is that thermal growth must be accounted for closely. Specifically, the thermal load in a combustor tends to be non-uniform. As a result, different parts of the combustor will experience different amounts of thermal growth, stress and strain. If the thermal combustor design does not account for non-uniform thermal growth, stress, and strain, then the usable life of the combustor may be negatively affected.
In many combustors, there is also a problem with damage to the combustor caused by vane bow waves. Failure to counteract got these vane bow waves also shortens the life of the combustor.